The invention relates to an operating circuit for a discharge lamp having electrodes which can be preheated.
It is known for the resonance of a resonant circuit to be used for the preheating mode of the operating circuit in discharge lamps in which electrodes are intended to be preheated. For example, the electrodes to be preheated may on the one hand be connected to a frequency generator in the operating circuit and may on the other hand be connected via a capacitor and optional further components to a preheating device. The preheating device thus contains a resonant circuit whose oscillations cause current to flow through the electrodes. When the operating device produces an oscillation in the resonant circuit, the electrodes are in consequence preheated. The preheating mode may be ended, for example, by the heating of a PTC thermistor.
In a prior German Patent Application with the file reference 101 02 837.7 (xe2x80x9cOperating device for discharge lamps with the filament heating being switched offxe2x80x9d), the applicant has already proposed an operating device in which a preheating transformer is used for the preheating process, which is carried out at a resonant frequency of a resonant circuit, to which the primary winding of the transformer is connected.
The present invention is based on the technical problem of specifying an operating circuit for discharge lamps having electrodes which can be preheated, which operating circuit has an improved preheating device.
The invention provides that the operating circuit is designed to produce an AC voltage at the start of operation, in the process to move through a frequency range which includes the resonant frequency of the resonant circuit and, in the process, to record the response of the resonant circuit by measuring an electrical variable such that the resonant frequency can be identified and the lamp can be preheated at this resonant frequency.
Advantageous embodiments are described in the dependent claims.
The invention is based on the fundamental idea, which has already been included in the cited unpublished patent application, of using a resonant circuit and its resonance for preheating. The invention is also based on an operating circuit, in which the operating frequency of the operating circuit can be varied and adjusted. The invention proposes that, at the start of operation, a search is made through a frequency range for the resonant frequency of the resonant circuit, which frequency range is chosen such that it can be reliably assumed that the resonant frequency can be found in this frequency range. The resonant frequency may, for example, be identified by determining the amplitude of a voltage value or of a current value. In this case, there is also no need to move through the entire frequency range and, in fact, the process of moving through this frequency range can be stopped once the resonant frequency has been found. For example, it would be possible to use rising voltage or current values and a decrease in these values once again to deduce that the process has passed through the maximum, and to define this maximum as the resonance peak.
The resonant frequency of the resonant circuit can thus be identified, and can be used for the subsequent preheating process. This makes it possible to ensure particularly efficient preheating, on the other hand excluding influences resulting from component tolerances or temperature fluctuations which, for example, may vary inductances.
A further advantageous option is to use the level of the detected amplitude at the resonance peak to deduce the type of discharge lamp being used. This is because, if the operating circuit is designed such that not only the operating frequency but also other operating parameters are adjustable, it can then be used for different lamp types. This procedure is particularly convenient if the operating circuit adjusts itself automatically to the lamp type being used. The lamp type can, of course, be detected by additional coding of the lamp. However, it is simpler and/or more convenient to use the technical characteristics of the lamp, which exist in any case, for identification. In particular, the resistances of the lamp electrodes in different lamp types differ. This results in different attenuations of the resonance, which can be detected and can be used to deduce the lamp type. The operating circuit can then set the appropriate operating parameters.
Identification of the lamp type may, in principle, be worthwhile even if only one lamp type is in principle envisaged. It is then possible to prevent a lamp type which fits mechanically but is electrically unsuitable for being inserted and operated. In this situation, the operating circuit could refuse to switch on if an incorrect lamp type were identified.
The use of a preheating transformer in the preheating device as has already been described in the cited unpublished prior application is preferred. The disclosure content relating to this, in particular with regard to the various connection options and embodiment variants for the resonant circuit, is hereby expressly referred to. In any case, two secondary winding of the preheating transformer should in each case be connected to one of the electrodes of the discharge lamp, in order to allow the discharge lamp to be preheated. Furthermore, the preheating transformer must be connected to the resonant circuit, with the resonant circuit preferably being located on the primary side, that is to say with the primary winding being connected to the resonant circuit. This allows the appropriate oscillations in the resonant circuit to be initiated by a frequency generator in the operating circuit without having to be transformed to the voltage level on the secondary side.
One advantageous option for detecting the response of the resonant circuit in order to identify the resonant frequency and, if necessary, also to determine the strength of the resonance for lamp type identification purposes is to measure the maximum amplitude of the voltage on the primary winding of the preheating transformer. To do this, this voltage is preferably rectified, as illustrated in the exemplary embodiment.
The frequency generator for the operating circuit is preferably in the form of a digital controller, which produces frequencies digitally. In this case, the frequency range can be moved through, according to the invention, in steps. To this extent, the appropriate frequency step which is closest to the resonant frequency is detected, rather than the actual resonant frequency itself. In principle, it is irrelevant to the technical function of the invention whether the resonant frequency is detected precisely. The aim is to use only the resonant peak for preheating purposes. Owing to the attenuation of the resonance as a result of the resistances of the electrodes, the resonance is in general not very narrow in any case, so that the aim is only to approach the resonant frequency approximately.
An advantageous order of magnitude for the resonant frequency is twice the operating frequency of the operating circuit in continuous operation of the discharge lamp. Typical orders of magnitude may, for example, be about 80-100 kHz for the resonant frequency and approximately 40-50 kHz for the continuous operating frequency.